1. Field
The present disclosure relates generally to the field of anti-reflective, anti-glare, barrier coatings applied to glass, metals and plastics in surfaces, windows, windshields, screens, displays, architecture, goggles, eyeglasses, etc. In particular to glass used as the front cover of solar modules, high-transparency glass used for display purposes such as protective covers for works of art; museum display glass; commercial display glass; front glass of electronic screens or instrument panels that need anti-glare properties and automotive glass.
2. Description of Related Art
Anti-reflective coatings are used in a wide variety of commercial applications ranging from sunglasses, windows, car windshields, camera lenses, solar panels, and architectural systems. These coatings minimize the reflections on the surface of the glass as the light rays travel through a discontinuous dielectric gradient. The reflection of light usually results in reduced transmittance of the light across the transparent material. For optical applications, a majority of incident light desirably passes through the interface for maximum efficiency. In this context, anti-reflective coatings provide a useful benefit in optical applications.
Anti-reflective coatings are normally used in glasses, acrylics, and other transparent materials that serve as windows and glass panels associated with architectural structures or energy generating and saving systems. In building windows, they are used to maximize influx of incident light to maintain proper lighting or natural ambience as well as to minimize distracting reflections from glass surfaces. In energy generating and saving devices, such as solar panels and light collectors, the utility of anti-reflective coatings lies in the enhanced efficiency of these devices due to a greater degree of light transmittance and, therefore, increased energy generation for the same cost.
In order for optical elements to perform optimally, it is necessary that they be free from surface contamination and depositions (e.g., dirt) that may reduce light transmittance and, therefore, performance of the coatings. In particular, for optical elements that are exposed to an outside environment, such as solar panels and building windows, the long term exposure to chemical and physical elements in the environment usually results in deposition of dirt on the surface of the optical element. The dirt may comprise particles of sand, soil, soot, clay, geological mineral particulates, air-borne aerosols, and organic particles such as pollen, cellular debris, biological and plant-based particulate waste matter, and particulate condensates present in the air. Over time, deposition of such dirt significantly reduces the optical transparency of the optical element. As a result, there is considerable expenditure of human and financial resources associated with regular cleaning of such optical elements, such as transparent windows and solar panels.
The deposition of dirt on such optical elements can be classified into two types: physically bound and chemically bound particulate matter. The physically bound particles are loosely held due to weak physical interactions such as physical entanglement, crevice entrapment, and entrapment of particulates within the nanoscale or microscale edges, steps, terraces, balconies, and boundaries on the uneven surface of the optical element, such as a window surface. These particles can be dislodged with moderate energy forces such as wind, air from a mechanical blower, or by means of water flow induced by rain or other artificially generated sources of flowing water such as a water hose or sprayer. On the other hand, chemically bound particles are characterized by the presence of chemical interactions between the particles themselves and between the particulate matter and the optical element itself, such as glass or acrylics (e.g., plexiglass) used, for example, in windows. In these cases, removal of these particles becomes difficult and usually requires the use of physical means such as high pressure water hoses or manual scrubbing or both. Alternatively, chemical means such as the application of harsh solvents, surfactants, or detergents to the optical element to free the dirt particles from the surfaces can be used.
As noted, the dirt on ambiently exposed optical elements, such as windows and solar panels, may be somewhat removed based upon natural cleaning phenomenon such as rain. However, rain water is only effective at removing loosely (physically) held particulate matter and is not able to remove the particulate matter that may be strongly (chemically) bonded to optical element, such as the glass or window surfaces. Furthermore, rain water usually contains dissolved matter that is absorbed from the environment during its descent that can leave a visible film when dried.
As such, all externally exposed optical elements, such as window materials and solar panels in which the optimal transmission of light is desired, require some form of routine cleaning efforts associated with their maintenance regimen. In fact, the surfaces of these items are cleaned during fabrication as well as routinely during use. The surfaces of these items, such as solar panels, are usually cleaned with water, detergent, or other industrial cleaners. As a result, anti-reflective coating materials applied to these optical elements need to be able to withstand the use of normal cleaning agents including detergents, acid, bases, solvents, surfactants, and other abrasives to maintain their anti-reflective effect. Abrasion of these coatings over time due to cleaning and the deposition of dirt or other environmental particulate may reduce their performance. Therefore, abrasion resistance is desirable for anti-reflective coatings. Resistance to abrasion is desirable in a coating used in connection with a solar panel, particularly for long term functional performance of the solar panel.
A majority of anti-reflective coatings are based on oxides as preferred materials. Some anti-reflective coatings are made of either a very porous oxide-based coating or, alternatively, are comprised of stacks of different oxides. These oxide materials are chemically reactive with dirt particles by means of hydrogen bonding, electrostatic, and/or covalent interactions depending upon the type of coating material and the dirt particle. Therefore, these oxide based coatings have a natural affinity to bind molecules on their surfaces. Further, highly porous coatings can physically trap dirt nanoparticles in their porous structure. As a result, current anti-reflective coatings are characterized by an intrinsic affinity for physical and/or chemical interactions with dirt nanoparticles and other chemicals in the environment and suffer from severe disadvantages in maintaining a clean surface during their functional lifetime.
Further, one of most common issue frequently associated with anti-reflective coatings is their performance over the entire solar spectrum, particularly with respect to solar panels. While there are several anti-reflective coatings that are only effective in a narrow region of the solar spectrum, for maximum efficiency it is desirable that anti-reflective coatings perform equally well over the entire solar region from 300 nm to 1100 nm.
Consequently, there exists a need in the art for a coating that can provide the combined benefits of anti-reflective properties, such as a coating that can reduce light reflection and scattering from the applicable optical surface; anti-soiling or self-cleaning properties, such as a coating surface that is resistant to binding and adsorption of dirt particles (e.g., resistant to chemical and physical bonding of dirt particles); abrasion resistant properties, such as stability against normal cleaning agents such as detergents, solvents, surfactants, and other chemical and physical abrasives; and UV stability or suitable performance over the entire solar region.
Further, it would be beneficial for such coatings to be mechanically robust by exhibiting strength, abrasion resistance, and hardness sufficient to withstand the impact of physical objects in the environment such as sand, pebbles, leaves, branches, and other naturally occurring objects. It would be beneficial for such coatings to also exhibit mechanical stability such that newly manufactured coatings or films would be less likely to develop cracks and scratches that limit their optimum performance, thereby allowing such coatings to be more effective for a relatively longer term of usage. In addition, it would be beneficial for such coatings to be able to withstand other environmental factors or conditions such as heat and humidity and to be chemically non-interactive or inert with respect to gases and other molecules present in the environment, and non-reactive to light, water, acid, bases, and salts. In other words, it is desirable to provide coatings having a chemical structure that reduces the interaction of the coating with exogenous particles (e.g., dirt) to improve the long term performance of the coating.
It would also be preferable to enable deposition of such coatings onto the optical surface, such as the surface of a window or solar panel surface, using common techniques such as spin-coating; dip-coating; flow-coating; spray-coating; aerosol deposition; ultrasound, heat, or electrical deposition means; micro-deposition techniques such as ink-jet, spay-jet, or xerography; or commercial printing techniques such as silk printing, dot matrix printing, etc.
It would also be preferable to enable drying and curing of such coatings at relatively low temperatures, such as below 150° C. so that the coatings could be applied and dried and cured on substrates to which other temperature sensitive materials had been previously attached, for example a fully assembled solar panel.
The front, sun facing surface of solar modules is often constructed of high transmission glass. This glass may be low-iron content soda-lime float glass or patterned glass between about 2 mm to about 4 mm thickness. Some solar modules may additionally use glass of a similar type on the back surface of the module. The outer surface of this type of glass will typically reflect about 4% of normally incident light because of the difference in refractive index of the glass, approximately 1.52, and the air approximately 1.0. In order to mitigate this loss of photons caused by reflection, the glass may be coated with an anti-reflective coating with an intermediate refractive index between about 1.25 and about 1.45.
Several methods exist to create anti-reflection coatings. Methods using silica nanoparticles have been known for a long time, for example U.S. Pat. No. 2,432,484 filed in 1943 teaches using a solution containing colloidal silica nanoparticles to create an anti-reflective coating on glass. Another more recent example is U.S. Pat. No. 7,128,944 which teaches a porous SiO2 layer created by depositing an aqueous solution containing silica particles that is then sintered at temperatures of at least 600° C. European Pat. Appl. EP1674891 teaches coating a substrate using hollow particles in a binder and then curing to create an anti-reflective coating. U.S. Pat. No. 8,557,877 also teaches using a combination of at least two different alkoxy silane materials in a base catalyzed solution as a coating that is cured at between 650° C. and 700° C. to create an anti-reflective coating. Multi-layer anti-reflective coatings are also well known in the art. For example, U.S. Pat. No. 4,387,960 teaches a 4-layer coating. One common distinguishing characteristic of these multi-layer coatings is that the outermost layer (adjacent to the air) has a lower refractive index when compared to the next layer in.
Many commercially available anti-reflective coating materials for the photovoltaic solar module industry utilize these or similar methods. A common feature is the requirement for a high temperature sintering or curing step at between 600° C. and 750° C. Since this processing step is also commonly used as a tempering step to mechanically strengthen the glass, both process are accomplished at the same time.
Anti-reflective coatings that are cured during the glass tempering process or by other similar high temperature processing share a number of common features. First, they are hydrophilic or super-hydrophilic, with water contact angles as measured by a goniometer of less than 60° and less than 20° respectively. The high temperature oxidizes all organic components of the coating, leaving behind almost pure silica. Second, they are frequently quite brittle. Their mechanical strength is derived to some extent by sintering of the coating. However, too much sintering reduces the coating porosity and its optical performance. At present, the balance achieved between optical and mechanical performance frequently leaves the mechanical performance at less than ideal. In general, these brittle hydrophilic coatings are prone to degradation caused by soiling and abrasion when subjected to long-term exposure in many outdoor environments.
It would be desirable if these high temperature coatings with high optical performance could be made more robust for both abrasion resistance and soiling resistance.